Among the many known methods of coating, one which is assuming increasing commercial importance is the method known as curtain coating. This method is characterized by the formation of a free-falling curtain of liquid coating composition. The object to be coated, for example a continuous web, or a series of discrete sheets carried by a conveyor belt or similar conveying means, is advanced through a coating zone and the coating apparatus is positioned within the coating zone above the path of the moving object. The falling curtain extends transversely of the path and impinges on the moving object to form the desired coating.
Many different types of apparatus are known for use in forming the free-falling curtain. For example, the curtain can be formed by use of apparatus utilizing an overflow weir, or by apparatus in which the coating composition is extruded from an elongated discharge slot, or by use of a slide hopper, or by use of a slide-extrusion hopper.
Regardless of the type of apparatus utilized to generate the free-falling curtain, curtain coating methods have in common the problem that the curtain is susceptible to disturbance by ambient air currents. The degree to which the curtain is susceptible to such disturbance is dependent, in part, on the height of free fall, with the susceptibility to disturbance increasing in more or less direct proportion to such height. In many instances, it is desirable that the height of free fall be relatively great, in order to provide a relatively high impingement velocity, and where the height is great the problem of disturbance is especially acute. For example, when an object to be coated, such as a continuous web, is advanced through the coating zone at high speeds, the barrier layer of air that it carries on its surface necessitates a substantial impingement velocity to achieve good coating results, and thereby necessitates a substantial height of free fall. Many other factors, in addition to the height of free fall, interact to determine the extent to which the curtain is susceptible to disturbance by ambient air currents; for example, important factors include the mass flow rate, the physical properties of the coating composition such as viscosity and surface tension, and the design of the coating apparatus.
In some coating operations, disturbance of the free-falling curtain by ambient air currents is not a serious problem. However, in other coating operations, such as the use of curtain coating in the coating of photographic materials, which require extremely precise conditions, it is a very serious problem indeed. Curtain coating is a very useful method for the coating of photographic films and papers, being well adapted to the application of both radiation-sensitive and non-radiation-sensitive layers. As described in Greiller, U.S. Pat. No. 3,632,374 issued Jan. 4, 1972, curtain coating can be used as a single-layer coating method in the manufacture of photographic materials. As described in Hughes, U.S. Pat. No. 3,508,947 issued Apr. 28, 1970, curtain coating is especially advantageous as a method of simultaneous multi-layer coating in which two or more layers of coating composition are simultaneously applied in the manufacture of photographic materials. Whether the curtain coating method is employed in the single-layer mode or in the multi-layer mode, the manufacture of photographic materials requires exacting conditions, so that disturbance of the free-falling curtain by ambient air currents is of great concern. To appreciate the extent of this concern, it should be realized that the mere opening and closing of a door to the coating room, or the movement of personnel in the vicinity of the coating apparatus, can cause severe curtain movement.
U.S. Pat. No. 3,632,374 to Greiller discusses the problem of selecting the height of free-fall in a curtain coating process as follows:
"In the practice of this invention, the height of the free-falling curtain, i.e., the distance over which free fall occurs, is selected to facilitate attainment of the desired objective of applying an extremely thin coating with extremely uniform thickness. In selecting the optimum height, an important criterion is that the height be made as small as is practical because the longer the free-falling curtain the more susceptible it is to being affected by ambient air currents causing flutter of the curtain and resultant non-uniformity in the product. However, the height must also be selected in accordance with the requirement that the free-falling curtain have adequate momentum at impingement to effectively penetrate or displace the air barrier and adhere to the moving support. To this end, it is desirable that the coating apparatus provide for adjustment of the height of free fall over a substantial range. The air barrier will vary with such factors as the character of the surface to be coated, the effectiveness of mechanical means utilized to remove entrained air, and the velocity at which the support is advanced. Also, since momentum is the product of velocity and mass, if the flow rate of the coating composition is reduced the height of free fall should, in general, be increased so as to increase the impingement velocity and give the free-falling curtain sufficient momentum to penetrate the air barrier. Under typical conditions in the practice of this invention, the height of the free-falling curtain will be in the range from about 5 to about 20 centimeters, but operation at smaller or greater heights than this is also fully within the contemplation of this invention."
It is well known to equip curtain coating apparatus with a shield to protect the free-falling curtain from disturbance by ambient air currents. For example, both U.S. Pat. No. 3,632,374 to Greiller and U.S. Pat. No. 3,508,947 to Hughes describe the use of a shield which is attached to the coating hopper and extends into close proximity with the path along which the object to be coated is advanced. Such shields are helpful, to a very limited extent, in protecting the free-falling curtain from disturbance by ambient air currents. However, they are much less effective than is desirable for optimum coating performance, and disturbance of the free-falling curtain remains a serious problem which hinders the successful employment of curtain coating in precision coating operations, such as photographic coating.
In bead coating operations, which at the present time are very widely used in the manufacture of photographic materials, disturbance by ambient air currents is also a serious problem. Bead coating is carried out by forming a bead of coating composition which is maintained in bridging relationship between the coating hopper and a surface of the web to be coated. Movement of the web across and in contact with the bead results in deposition of a layer of coating composition on the web. Bead coating is useful both as a single layer coating method and as a method in which the bead is formed from a plurality of flowing layers to thereby carry out simultaneous multi-layer coating (See, for example, U.S. Pat. Nos. 2,681,294, 2,761,417, 2,761,418, 2,761,419 and 2,761,791). A particularly useful type of coating hopper, for carrying out a bead coating operation, is the slide hopper. Such hoppers comprise one or more slide surfaces down which a layer of coating composition is flowed in forming a coating bead. However, a serious difficulty occurs in the use of slide hoppers in that the coating composition flowing down the slide surface is exposed to contact with ambient air currents. This can result in differential evaporation of the liquid medium from the coating composition as it travels on the slide surface and, as a consequence thereof, the formation of mottle or other defects in the coating.
Slide hoppers are also advantageously employed in both single layer and multiple layer curtain coating operations. In these processes, differential evaporation on the slide surface is also a significant problem. Accordingly, it is desirable in such processes to protect the coating composition against disturbance by ambient air currents both when it is travelling down the slide surface and when it is undergoing free fall.
It is toward the objective of providing improved means for protecting a flow of coating composition--such as a free-falling curtain or the flow on the slide surface of a slide hopper--from disturbance by ambient air currents that the present invention is directed.